Game theory explains why some content goes viral on Reddit, Digg

Two game theorists have shown in a recent paper that it may not only be …

A lot of attention has been lavished on ideas "going viral," but this may not be the only way that ideas spread, according to an article published in PNAS last week. With some extensive theoretical work in game theory, two researchers have shown that trendy changes don't spread quickly just because they gain exposure to a high number of people. Instead, the spread of innovations may work more like a game where players are gauging whether to adopt something new based on what others immediately surrounding them do.

The popularity growth of things like websites or gadgets is often described as being similar to an epidemic: a network with a lot of connections between people increases exposure and then adoption, as do links stretching between dissimilar groups. When the trend in question spreads to a node with a lot of connections (like a celebrity), its popularity explodes. While this is fitting for some cases, in others it's an oversimplification—a person's exposure to a trend doesn't always guarantee they will adopt it and pass it on.

"It is not only the intrinsic value of a new technology (or other types of innovation) that makes it attractive. It is also the number of friends who have adopted it," Amin Saberi, one of the authors, told Ars. In instances where there is incentive to make the same decision as people around you, the authors of the paper argue, the spread of innovations may instead follow rules of game theory, which differ in big ways from the rules of viral or epidemic trends.

To demonstrate how this is possible, the two researchers set up a theoretical scenario with several people, or nodes, connected in a network, like friends in a social group. They then instituted a game where in each round, each node had to decide whether to adopt a new innovation based only on the current behavior of their neighbors.

For example, a node/person in the game would look around and see how many of his friends were participating in a trend, say, Farmville. If none were, the odds of the node starting to play Farmville were low; if all were, odds of playing Farmville were high. The game was weighted so that imitating neighbors' behavior had a higher payoff than going against the grain.

With only these rules, a social enclave where everyone has perfect information about what everyone else is doing would never adopt anything new. If people only made decisions based on that others were doing, none of the nodes would ever see changing as the best strategy.

To fix this, the researchers introduced some noise into the situation so many nodes had incomplete information. They weighted the decision so that a node with zero information about what his neighbors did would choose to adopt the innovation, whatever it was (this could be considered an analog to a reality where a person doesn't care what others think and evaluates new innovations based on other factors).

When they played around with the structure of the network operating on these rules, they found nodes with local connections, as opposed to the long-range ones that facilitate epidemics, spread innovations more quickly. Nodes that weren't as tightly integrated to the network and maintained fewer connections let change spread more quickly, while nodes with lots of connections actually slowed the spread down.

The highly connected nodes turned into roadblocks, because even without perfect information on its neighbors' outlook, the highly connected nodes get more external pressure from their unenlightened neighbors. A highly connected node must then be extremely ignorant of its neighbors to adopt a trend, or else must be surrounded by neighbors that have switched over first. This was one of the biggest differences between the game-theoretic spread and the epidemic spread.

The model seems to apply less to individual pieces of content, where simple exposure is enough to create huge growth. On the other hand, it could explain, for instance, loyalty to sites that distribute that content, like Digg and Reddit, or to particular genres of memes. The authors say it also crops up in choices that influence social connections, like the choice between voting Republican or Democratic, or to adoption of technology, like choosing between Verizon and AT&T.

Dr. Saberi gave the following example: "the reason I am using Facebook as opposed to another social network is not just its quality it is also because I have a lot of friends who are using it"; he notes this could also apply to operating systems. Likewise, while there are many reasons to choose one cell phone carrier or another, features like free calls or texts within a network can influence a group of friends to migrate to the same network as each other.

In the game theory model, networks trend to an equilibrium of everyone adopting the change—not terribly realistic. Still, the model shows that trends may spread quickly based on something other than the brute force of exposure. Even with a more complex, socially influenced process, the popularity of an innovation can grow rapidly.

Casey Johnston
Casey Johnston is the former Culture Editor at Ars Technica, and now does the occasional freelance story. She graduated from Columbia University with a degree in Applied Physics. Twitter@caseyjohnston

30 Reader Comments

Hmm, they should repeat the experiment with repeated rounds. So that people reevaluate on later rounds. They should also include an influence model where some nodes more heavily weight the decision of other nodes.

@LetterRip: I think this couldn't have been modeled without simulating repeated rounds. The problem here is obviously that due to the specific payoffs, their findings describe a definitional equilibrium. Without dynamically altering payoff structures, this simulation is likely to lead to the same outcome everytime it's run.

I'm fairly certain that consistent outcomes are the point of modeling, even with dynamically altering payoff structures as if all the variables are properly being controlled for, running the same experiment twice should see the payoff changing the same way both times.

I would believe that it may be appropriate to summate these findings with the following statements: The more popular something is, the more people will be drawn to using it. If something can be "endorsed" by influential people in a social circle, it's chances of being used by those who are influenced is higher.

To me this sounds too complicated. Is virality not the product of 4 simple rules?

1. If it's not too much trouble, people will experiment with anything that looks interesting.2. Of the things they try, people only use the things they like.3. Of the things they like, people will only recommend things that they *really* like and that could make for interesting conversation. E.g. toilet paper unlikely to go viral, no matter how amazing one's arsehole feels after using it.4. If they get bored of the thing, people stop both recommending and using. If everyone knows about the thing, people stop bothering to recommend no matter how much they like it - ever heard a friend mention how you should drink more Coke because it's so great?

Bottom line: make something very worth talking about and lots of people will talk about it - no brute exposure or social influence models necessary.

Is viral even possible now on Digg? Since the content is artificially created through RSS feeds now and paid for links are propped up on the front page, I don't even think the new Digg is even part of the equation, generally users just vote on what is laid in front of them now rather than their own content.

I just wait for marketing to gain a expert system that can take a suggested ad and predict its ability to spread and generate sales. Then we see custom crafted meme ads that will once and for all confirm that free will is a very big illusion unless one fully disconnect from the social world.

I think predicting the probability something is viral is not as easy as some commenters might think... This study helps try to discover what it is that makes people want to share information, and one factor is that it depends on how many people you are connected to.

It seems to me that well connected people influence virility much more, even if less connected people are faster at propagating trends. Seems interesting to me. I wonder if there are common trends between humor preferences and number of connections, for example.

They needed a scientific study for this? ONe could show the same thing just looking at WoW. I don't know how many people I know play it, even though they hate it and it frustrates them, simply because everyone else they know play it and they want to be apart of what their friends are apart of. We are societal beasts so it only goes that whatever those in your clique go to, whether that be iPhones, Kindles, WoW, or American Idol that too will be most likely what you are going to do. Just like Oprah, whatever she pushes turns to gold because everyone wants to do the in thing. I don't know that that is as much game theory though as it is just societal expectations given the type of people we are. Easily swayed by emotions and peer pressure so we can stay up with the Joneses, and that has been known by marketers for awhile now.

Dr. Saberi gave the following example: "the reason I am using Facebook as opposed to another social network is not just its quality… it is also because I have a lot of friends who are using it." ... the model shows that trends may spread quickly based on something other than the brute force of exposure.

Thank god we now have a mathematical model telling us something which was patently obvious to begin with.

The authors say it also crops up in choices that influence social connections, like the choice between voting Republican or Democratic, or to adoption of technology, like choosing between Verizon and AT&T.

This same reasoning can be applied to trends in general, not just social ones. The foods restaurants serve (in particular the "in-ness" of certain ingredients) often follow these novelty trends as well.

The real way this needs to be tested is to make something that is verifiable crap (and has documentation to prove it was created with such an intent) and pay people to pretend to like it to measure the growth of the trend. The cynic in me says this experiment has been done numerous times and was found too profitable to ultimately divulge to the people.

The real way this needs to be tested is to make something that is verifiable crap (and has documentation to prove it was created with such an intent) and pay people to pretend to like it to measure the growth of the trend. The cynic in me says this experiment has been done numerous times and was found too profitable to ultimately divulge to the people.

The greater cynic in me doubts whether ultimately divulging it to the people would make any difference at all. I strongly suspect that, no matter what evidence was presented regarding why/how it was designed, devoted Mac fans would still defend the Mac.

And you can feel free to substitute in PS3, 360, Republican, Democrat, or the religion of your choice for "Mac," there.

They needed a scientific study for this? ONe could show the same thing just looking at WoW. I don't know how many people I know play it, even though they hate it and it frustrates them, simply because everyone else they know play it and they want to be apart of what their friends are apart of. We are societal beasts so it only goes that whatever those in your clique go to, whether that be iPhones, Kindles, WoW, or American Idol that too will be most likely what you are going to do. Just like Oprah, whatever she pushes turns to gold because everyone wants to do the in thing. I don't know that that is as much game theory though as it is just societal expectations given the type of people we are. Easily swayed by emotions and peer pressure so we can stay up with the Joneses, and that has been known by marketers for awhile now.

Surely this must be a common sentiment, but I still think this paper is good news(even though this has been at least theorized for a while). Personally I think the fact that humans are conformist by design and that everyone will deterministically follow their neighbors is a fairly depressing understanding of human nature. The reason I appreciate this paper is that one can explain why it is actually rational (acting in one's self-interest) to "conform". It's not just that we're all doomed into doing what everyone else does, it's actually in our self-interest to copy others, in certain cases. There goes one more thing we can cross off on the list of humanity's fundamental flaws.

Several comments here [paraphrased] correctly ask the question - "I need game theory to tell me why some content is popular and other is not? Seems like overkill." I agree it is overkill. Metcalf"s Law has taught us all the value of the network, like Reddit and Digg.

But don't be so fast to dis game theory in this primitive example. It is a primitive example because all the players in the game are homogenous and only have two options available to them i) use a social bookmarking service ii) dont use a bookmarking service. The application of game theory is far more interesting when used to predict outcomes involving multiple players each acting in their own self-interest and having many and varying options available to them. Predicting outcomes in markets riddled with new players, consolidation, obsolescence, and innovation is a great application for game theory.